1. Field of the Invention
The present invention is concerned with a method for producing amorphous cordierite-forming glass from an organometallic precursor. Amorphous cordierite (Mg.sub.2 Al.sub.4 Si.sub.5 O.sub.18) formed by the method of the present invention can be used to produce thin ceramic films or sheets which are useful in forming multilayer electronic devices. The present invention also provides a method f producing amorphous compositions comprising two major components wherein one of the two major components is an oxide of silicon.
2. Background Art
The synthesis of glasses and metal oxides from organometallic precursors (often referred to as sol-gel technology) has been of great interest to the ceramics industry in recent years. The first organometallic-derived metal oxides were produced about 45 years ago as coatings; see Reichspatent 736411 (1939) to W. Geffcken and E. Berger. In 1971, H. Dislich disclosed that multicomponent oxides could be synthesized by hydrolysis of metal alkoxides by the sol-gel process, in which the alkoxides are dissolved in an organic solvent, reacted with water, and condensed to a fine particulate oxide; see H. Dislich, Angew. Chem. Int. Ed. Eng. 10, 363 (1971). Refractory oxides derived from organometallic precursors, such as stabilized zirconia from alkoxides have been synthesized by K. S. Mazdiyasni et al., as described in both K. S. Mazdiyasni et al., Journal of the American Ceramics Society 50, 532 (1967) and K. S. Mazdiyasni, Ceramics International 8, 42 (1982).
The synthesis of metal oxides is not confined to degradations of alkoxide precursors. The amorphous citrate process, in which metal citrates are mixed in aqueous solution, dehydrated to a gel, and pyrolyzed to an oxide has been demonstrated for synthesis of numerous metal oxides. See: Ph. Courty et al., "Mixed Oxides and Oxide Solid Solutions in Highly Dispersed Form Obtained Through Pyrolysis of Amorphous Organic Precursors, Powder Technology 7, 21 (1973); M.S.G. Baythoun et al., "Production of Strontium-substituted Lanthanum Manganite Perovskite Powder by the Amorphous Citrate Process", Journal of Materials Science 17, 2757 (1982); and D. J. Anderton et al., "Production of Conducting Oxide Powders by Amorphous Citrate Process", Powder Metallurgy 1, 14 (1979). In addition, Hirano has developed a synthesis of spinel ferrites from metal acetylacetonates, which is in pilot scale production in Japan; see S. Hirano et al, "Preparation and Properties of Mn,Zn-Ferrite Fine Particles by Hydrolysis of Organo-Metallic Compounds", American Ceramic Society Bulletin 61, 362 (1982) and Y. Suwa et al., "Preparation of Spinel Ferrites by Hydrolysis of Metal Acetylacetonates" Proceedings of the International ICF, 3rd (48TRAI) 1980 (Pub. 1982), 23-26.
Ferrimagnetic spinels have been synthesized by both the hydrolysis of metal acetylacetonates (See U.S. Pat. No. 4,486,401 to Arons et al.) and by the amorphous citrate process (See U.S. Pat. No. 4,473,542 to L. D. David).
The advantages of synthesizing ceramics from organometallics are: (1) Ultrafine, submicron particles, with extraordinary surface area and sinterability can be obtained using specialized process conditions. (2) Syntheses take place at low temperatures, as much as 1000.degree. C. lower than in conventional processes. This suppresses gross grain and particle growth, phase separations, and losses of volatile oxides. Kinetically stable phases not possible using other methods of synthesis become accessible. (3) Molecular level mixing is achieved by the mixing of molecular precursors in solution prior to the hydrolysis step (in the hydrolysis of metal alkoxides, acetylacetonates, etc.). Thus, homogeneous mixed oxides are much more easily made. (4) Multicomponent oxide compositions not obtainable by mixing, ball-milling, and melting together thermally incompatible starting materials can be achieved by organometallic precursor methods. (5) Amorphous or ultrafine crystalline ceramics are desirable in that sintering of the powder can be induced at temperatures far below that which would cause catastrophic grain growth. Organometallic-derived ceramic powders fill this need.
There is a need for methods of synthesis of fine, equiaxed, homogeneous particles of cordierite-forming glasses of both the .alpha. and the .mu. phases. Such powders are desirable for thin film redistribution applications in glass-ceramic substrate packaging of electronic devices and for insulation applications in semiconductor structures in general.
The synthesis of multioxide compositions, contrary to some assertions in the literature, is extremely complex. Areas of particular difficulty include the following:
(1) The burnout of carbonaceous residues must occur before pore closure in the incipient glass-ceramic. Gels derived with a cordierite target composition can act like molecular sieves in reforming organic products of hydrolysis. Oxidation can manifest itself as a dehydrogenation of a carbon-oxygen bond, forming an aldehyde, which decarbonylates to carbon monoxide and an alkane. The carbon monoxide can then disproportionate on the catalytic oxide surface to carbon plus carbon dioxide, and the alkane can conceivably dehydrogenate as well. Any synthesis of cordierite must suppress this carbon formation. Carbon residue can adversely affect the dielectric constant of the ceramic. It can also act as a standoff between individual particles, inhibiting sintering.
(2) Multicomponent precursors may hydrolyze at different rates, leading to phase separations in the dial product. One synthesis approach has been to hydrolyze double alkoxide precursors to homogeneous two-component perovskites and spinels. Another approach has been to utilize a mixed acetylacetonate-alkoxide precursor, as Hirano has done in ferrite syntheses.
(3) W. Holand et al., Journal Non-Crystalline Solids 48, 205 (1982), reported that during an attempt to synthesize cordierite from organometallic precursors, phase separations occurred above 600.degree. C. when the product was held for 24 hours at those temperatures. The reported synthesis employed alkoxide precursors of SiO.sub.2 and Al.sub.2 O.sub.3 and magnesium acetate as the MgO source. This alkoxide approach is therefore unsuitable for making cordierite-forming glasses for thin film redistribution (TFR) applications.
Y. Ozaki, Japanese Kokai Patent No. Sho 57(1982)-88075, reported the synthesis of cordierite from organometallic sources. Separate organometallic-derived sols of magnesium, aluminum, and silicon oxides were mixed, agitated, and cast into thin films. These films, however, passed through the spinel-.beta.-cristobalite phases between 600.degree. and 1200.degree. C., and did not transform to cordierite until above this temperature. Only at 1300.degree. C. was the single-phase cordierite obtained This high temperature was not much of an improvement over current melt-synthesis temperatures (1600.degree. C.), and firing a glass-ceramic/copper conductor system, typical of those used for packaging in the semiconductor industry, at that temperature is not feasible either.
It is known in the art, as described in U.S. Pat. Nos. 4,234,367 to L. W. Herron et al., 4,301,324 to A. H. Kumar et al., and 4,340,436 to D. J. Dubetsky, that the addition of boron and phosphorous to the cordierite crystalline structure alters the coefficient of expansion of the structure. The crystalline structure absent boron and phosphorus is .mu. phase, and presence of these elements results in the formation of .alpha. phase structure. The coefficient of expansion of the .mu. phase cordierite is about 33.times.10.sup.-7 /.degree.C. to about 35.times.10.sup.-7 /.degree.C., whereas the coefficient of expansion of pure .alpha. phase crystalline cordierite is about 5.times.10.sup.-7 to about 7.times.10.sup.-7 /.degree.C.
The coefficient of expansion of .alpha. cordierite-containing ceramic produced by the method of the present invention, described below, is about 25.times.10.sup.-7 /.degree.C. due to the amount of glassy phase present. Since this latter coefficient of expansion is very close to the coefficient of expansion of silicon (about 25.times.10.sup.-7 /.degree.C.), there are obvious advantages to use of this latter form of .alpha. phase cordierite in semiconductor applications.